Comparative studies on Na-dependent Pi transport in ovine, caprine and porcine renal cortex

Abstract

In contrast to monogastric species, renal excretion of inorganic phosphate (Pi) in ruminants is low and this could be attributed to an almost complete tubular Pi reabsorption. However, the functional and regulatory basis for this phenomenon has not yet been clarified. Therefore, it was the aim of the present study to characterize the kinetic parameters of the tubular Pi reabsorption system as affected by P or Ca depletion using preparations of renal cortex brush border membrane vesicles (BBMV) from goats and sheep and to compare the data with respective parameters of porcine preparations. Na-dependent Pi uptake into renal cortex BBMV as a function of Pi concentration showed typical Michaelis-Menten kinetic and respective Scatchard plot analysis of the specific Pi uptake revealed linearity indicating the predominant presence of a single type of Pi transporters in the preparations. Under control conditions Vmax values of Na-dependent Pi uptake into BBMV were highest in goats and sheep and lowest in pigs (1.98, 1.39 and 0.95 nmol x mg(-1) protein x 10s(-1), respectively). Km values were not different between goats and sheep under all feeding conditions and ranged from between 0.34 mmol x l(-1) and 0.55 mmol x l(-1) which was three- to five-times higher than that found in pigs (0.11 mmol x l(-1)). Oligonucleotides derived from rat kidney cortex type IIa Na/Pi cDNA were used for reverse transcriptase-polymerase chain reaction (RT-PCR) in goat, sheep and pig kidney cortex. The products isolated were 768 bp for sheep and pigs and 765 bp for goats, with the respective amino acids sequences, representing a segment of approximately 40% in length of the entire transporter, exhibiting an at least 92% sequence homology between different species. From the results, involvement of type IIa Na/Pi cotransport in tubular Pi reabsorption in small ruminants can be postulated. However, it should not be considered that a potential role of other Pi transport systems be completely be excluded. Interestingly, neither P nor Ca depletion caused significant effects on Na-dependent Pi transport capacities and affinities in goats and sheep. From this, parathyroid-hormone independent regulatory pathways of tubular Pi reabsorption can be assumed.

Review: Regulation of gastrointestinal and renal transport of calcium and phosphorus in ruminants

“…Interestingly, a stimulation of NaPi IIa expression was accomplished by dietary protein reduction and thereby, presumably, a reduction in P i in the ultrafiltrate. A direct dietary P i depletion without manipulation of GFR did not show the same effects (Schröder et al, 2000).…”

“…The uptake of filtered P i at the apical side is mediated by Na þ -dependent P i transporters: electrogenic NaPi IIa (SLC34A1) and electroneutral NaPi IIc (SLC34A3) in both ruminant and non-ruminant species (Figure 7) (Biber and Murer, 1994;Shirazi-Beechey et al, 1996;Huber et al, 2003;Starke et al, 2013). Ovine and caprine amino acid sequence, kinetic and stoichiometric parameters of renal cortex Na þ -dependent P i transport are comparable to the type IIa Na þ /P i co-transport in monogastric species (Schröder et al, 2000). Basolateral P i extrusion mechanism is still unknown.…”

“…Changes in dietary P intake and consequent changes in extracellular P i and PTH are the main regulators of renal P i transporters in monogastric species (Biber et al, 1998). In mature goats and sheep on a P-reduced diet, no changes in renal transport capacities (Schröder et al, 2000) or on NaPi IIa expression (Huber et al, 2007) were determined. In contrast, dietary P restriction altered urinary P i excretion in goat kids (Table 1).…”

Expression of Phosphatonin-Related Genes in Sheep, Dog and Horse Kidneys Using Quantitative Reverse Transcriptase PCR

“…Plasma uric acid concentrations in horses increase with intense exercise [ 34 ] and endurance exercise [ 35 ], and perhaps high renal NPT1 expression may assist the horse with removal of uric acid produced during exercise. Similarly, a study examining the kinetic parameters of sodium-phosphate co-transporter IIa in sheep and goats found that dietary phosphorus or calcium deficiency did not alter NPT2a transport capacity or affinity [ 36 ]. The authors suggested that PTH-independent pathways for phosphorus reabsorption may be important in the kidneys of ruminants, perhaps suggesting a role for NPT1 in renal tubular phosphate transport in sheep and goats.…”

Phosphorus utilization in domestic animals

“…Compared with swine, the Na-dependent P transporter has higher and K m (Schroder et al 2000). Furthermore, the plasma Pi concentration that reaches the maximal tubular reabsorptive rate (Tm) is 3 to 4 times above the normal physiological range, which explains that 99% filtered Pi is reabsorbed (Widiyono et al 1998).…”